1. Field of the Invention
The present invention relates to an optical power device protector and an optical transmission system using it. More particularly, the present invention relates to a protector for an optical power device designed to transmit light through optical fibers and an optical transmission system using such protector, and a method for protecting optical power devices.
2. Description of the Related Art
In today's fiber-optic transmission systems using optical fibers, WDM (Wavelength Division Multiplex) technology is used extensively in order to keep pace with the rapid increase in communication traffic. When this technology is used, the level of incoming light into optical fibers depends on the number of multiplexed wavelengths. In other words, the power level of light that is launched into optical fibers becomes considerably high as the number of multiplexed wavelengths increases. As a result, high-power optical amplifiers that are capable of outputting light whose level is in the previously-unimaginable watt range are being commercialized and gradually spreading.
In addition, the development of a technology for amplifying light, which is called distributed Raman amplification, is also being actively promoted with a view toward commercialization. Raman amplification is an amplification process that utilizes a mechanism, wherein launching signal light and pumping light, whose frequency is approx. 13 THz higher than signal light, simultaneously into an optical fiber made of silica glass causes part of the energy of pumping light to move into signal light due to the induced Raman scattering phenomenon that occurs in silica glass.
Distributed Raman amplification refers to a form of transmission wherein pumping light is launched into an optical fiber itself that is transmitting signals, by which signal light is amplified while it is transmitted. Propagation loss in a transmission line is thus compensated by this Raman amplification, and the accumulation of noise light caused by optical amplification relaying is suppressed, allowing the transmittable distance to be extended. In order to realize distributed Raman amplification, it is necessary to launch high-power pumping light of almost one watt into a transmission line optical fiber.
A similar method of inputting high-power light into a transmission line fiber is an optical amplification system based on remote pumping. A typical fiber-optic amplifier has a constitution wherein optical fibers doped with a rare earth element, e.g., erbium (Er) are excited in advance by inputting pumping light, and signal light is passed through these fibers to induce emission for amplification. Remote pumping refers to a constitution wherein a transmission line optical fiber is intercalated between an optical fiber doped with a rare earth element and a source of pumping light.
An advantage of this constitution is that transmission distance can be extended by the length of intercalated transmission line optical fibers used; a disadvantage is that the input power of pumping light must be made larger by the amount of loss caused by the intercalation of the transmission line optical fiber. The power of excitation has also reached the 1 W mark in recent years.
As stated above, in fiber-optic transmission systems using optical fiber, the level of power launched from an optical power device into optical fibers has been trending toward an increase, generating concerns that various problems may arise from use of such high-power light.
Many cases have been reported in which a level of light, which could be transmitted through an optical fiber cable without causing any problems in the cable itself, caused the optical connectors and other end faces of optical fibers to be burnt out. This problem cannot occur if an optical fiber cable is entirely fusion-spliced; since there is no optical connector in such a cable, the end faces of an optical fiber are not exposed and thus there is no possibility of their being burnt. Fusion-splicing an optical fiber cable entirely, however, is not a realistic practice, and cables are mostly connected with each other using optical connectors.
The problem of burnout of optical connectors and other elements has been known since when optical amplifiers was first commercialized. Many efforts have been made to eliminate this problem through various approaches, including replacing conventional materials for optical connector and other parts with wear-powder-free materials, improving the method of cleaning the end faces of optical fibers, and many others. These efforts, however, have not led to the complete elimination of burnout of optical fiber end faces. To the contrary, this problem is being experienced even more frequently as the power level of light has been heightened.
Another problem, which is still more serious, is a phenomenon called “fiber fuse,” wherein burnout of an end face of an optical fiber triggers a chain reaction of burnout, causing the symptom to be propagated through the cable toward the optical power device. If the heat generated while an optical fiber end face was burning should cause the temperature of the optical fiber to exceed approx. 1,100 degrees C., a vicious circle occurs. That is, the absorption coefficient for the glass in the optical fiber increases rapidly to the extent that the heat is transferred through the optical fiber to reach the adjoining optical fiber. In turn, the absorption coefficient for the glass in the adjoining optical fiber increases, and so on.
The fiber fuse (meltdown) phenomenon, which is one of the problems that are concerned about in relation to inputting higher-level light into optical fibers, is reported in greater detail in SPIE Vol.2966, pp.592-606, “A comparative evaluation of fiber fuse model,” and SPIE Vol.2714, pp.202-210, “Experimental data on the fiber fuse,” both by D. D. Davis, S. C. Mettler, and D. J. DiGiovanni.
Once it occurs, the fiber-fuse phenomenon easily gets over the fuse-spliced parts of optical fibers, the connections of optical connectors, and other sections, ultimately causing damage to reach the inner part of the optical power device. Considering the difficulties expected in repair of an optical power device internally destroyed by the fiber-fuse phenomenon, it is critical to prevent the fiber-fuse phenomenon from progressing before it reaches the optical power device.